The invention relates to a method for fabricating photodiode responsive to blue light.
Silicon photodiodes and, it relates, more particularly, to the fabrication of photodiodes adapted to the highly for the wavelength range of approximately 250 nanometers (nm) to 500 nm must have a very flat pn junction since it is known that the penetration depth of short-wave radiation in the semiconductor crystal is very small. For a wavelength, .tau.=400 nm, the penetration depth d is, for example, approximately 0.2 microns (.mu.).
With the aid of diffusion processes, the problem of realizing very flat pn junctions is very difficult particular with respect to being be reproducibly controlled.
When using ion implantation methods, the formation of very flat dopings is relatively simple. But the fact is that the maximum of the implantation doping concentration does not lie on the surface of the crystal and is somewhat deeper is a disadvantage. The charge carriers freed by the energy of light, therefore encounter a concentration profile which is directed oppositely to their nominal direction of diffusion to the pn junction.
One way of avoiding this is the utilization of a scattering medium in the form of an oxide, nitride, .alpha.-silicon etc., with the demands for maintaining the proper thickness of the layer or penetration depth being great.
A further possibility for locating the implantation peak on the semiconductor crystal surface is to etch off the topmost or outer silicon layer. Due to the small thickness and the required uniformity this is not possible with normal etching methods customarily used for removing semiconductor material.